1. Field of the Invention
This invention relates to a receiver that uses Galileo satellites in position determination. More particularly, it relates to a receiver that uses combinations or all of the codes in the AltBoc signals transmitted by these satellites to increase the precision of position determination by ground-based, airborne and seagoing receivers.
2. Background Information
The European Space Agency has designed and begun deployment of orbiting satellites to be used, inter alia, for position determination. The Galileo system is described in a number of publications, including Technical Annex to Galileo SRD Signal Plans, Draft 1, Jul. 18, 2001, ref # StF-annex SRD-2001/003 and the Galileo Open Service signal In Space Interface Control Document (OS SIS ICD) Draft 0 and later drafts published by the European Space Agency. However, a cursory review of the signal format for position determination will be of help in understanding the invention. The signals are generated in two adjacent frequency side bands, E5a and E5b. The E5a side band consists of a data-carrying channel modulated with a pseudorandom code (PRN) C0 and a dataless pilot channel modulated with a pseudorandom code C2. In effect, these two codes modulate a carrier whose frequency is 1176.45 MHz. The code C0 modulates an in-phase version of the carrier and the code C2 modulates a quadrature version.
The E5b signal consists of a data carrying channel modulated with a pseudorandom code C1 and a pilot channel modulated with a pseudorandom code C3. In effect, these two codes modulate a carrier whose frequency is 1207.4 MHz. The code C1 modulates an in-phase version of the carrier and the code C3 modulates a quadrature version. The data modulated on C1 does not necessarily have the same bit rate as the data modulated on C0, although the bit transitions coincide. The number of chips in each PRN, or spreading, code is 10,230 and each code is generated at a rate of 10.23×106 chips per second.
The modulation in each of the channels is by binary phase shift keying (BPSK). The four spreading codes C0 . . . C3 are combined into an AltBoc signal i.e., a BOC (10, 15) signal designed as an 8PSK signal to maintain a constant output power. The carrier frequency of this signal is 1191.795 MHz, midway between the E5a and E5b carrier frequencies.
The data is modulated onto the C0 and C1 codes by multiplication, resulting in:C0*=C0×NAVE5a andC1*=C1×NAVE5b 
A subcarrier (or “splitting subcarrier code”) in the form of a square wave, having a chip rate of 15 MHz (actually 15×1.023×106), is mixed with the four codes in a unique way to form the AltBoc (10, 15) combined spreading code.
Specifically, the phase angles of an AltBoc 8PSK signal are generated in accordance with the lookup table shown in FIG. 1, as applied to the RF phase polar chart of FIG. 2. The chart has eight points, each of which indicates a carrier phase displaced by 45 degrees from its adjacent points. The inputs to the table are the pseudorandom codes (C*0, C*1, C2, C3). T0 . . . T7 are time intervals, the duration of each interval being ⅛ of the period of the splitting subcarrier code, and thus, the table corresponds to one wavelength of the 15 MHz splitting subcarrier code. A code combination of, for example, (C*0, C*1, C2, C3)=(1, 1, 1, 1) produces an output of (1, 1, 5, 5, 5, 5, 1, 1) during the respective time intervals T0 . . . T7, these values being the like-numbered points on the polar chart.
The angle (MMT p) and offset (MMToff) columns in FIG. 1 represent a condensed version of the columns [T0 . . . T7]. The angle represents the starting point in the polar chart of FIG. 2 and the offset represents the splitting subcarrier code phase transition time from the center of the corresponding line in the table. For example, an angle of 1 and an offset of 2 (1,2) translates to a signal switching back and forth between points one and five (180 degrees) in FIG. 2, with the transition points at time at T2 and T6 yielding the series [1, 1, 5, 5, 5, 5, 1, 1]. Note that 4 time intervals are ½ wavelength of the 15 MHz splitting subcarrier code. The code is also sometimes referred to herein as “the splitting code.”
Note also that the modulus of the 15 MHz splitting subcarrier code timing is used as a lookup index in FIG. 1. For example, the TS lookup index of range [0 . . . 7] to use at any time T will be Ts=INT (T*8*15×1.023×106) MOD 8. Sampling in this fashion accomplishes the splitting subcarrier code multiplication and results in bi-directional rotations of the spreading codes phases at a rate of 15 MHz. Where the C0 and C1 signals end up at 1176.45 MHz, offset −15.345 MHz from the AltBoc center frequency of 1191.795 MHz. Similarly, the C2 and C3 signals end up 15.345 MHz higher at 1207.14 MHz.
FIG. 3 is a simplified diagram of a circuit arrangement that might be used to generate the AltBoc signal transmitted from a satellite. As shown therein, the C0 and C1 inputs are multiplied by the respective NAV data signals and applied to an AltBoc lookup table (e.g. FIG. 1) along with the C2 and C3 inputs. The table converts the four inputs into the outputs reflected in the table, and these are applied to an 8PSK modulator 20 that shifts the phase of the output of a 1191.795 MHz carrier generator. The output of the modulator 20 is the AltBoc signal.
The receiver (not shown), operating in a known manner used, for example, by conventional GPS receivers, correlates incoming AltBoc signals with local code and carrier generators of either the E5a code or the E5b code. The correlation with, for example, the in-phase components of the E5a code recovers the NAV E5a data, while the E5a quadrature phase (pilot) components are used for phase tracking and pseudorange determination. Specifically, the I and Q E5a correlation outputs are combined in a feedback loop to align a locally generated code and carrier with the corresponding components of the E5a portion of incoming signal. The amount of phase shifting required to align the local codes to the received codes is used in a well known arrangement to calculate the pseudorange to the satellite.
The position determination obtained by the Galileo system is materially more precise than that of prior GPS C/A code receivers. This is partly due to the use of a higher chip rate in the spreading codes and the use of a greater bandwidth.
The present invention is directed to an improvement of the accuracy of the pseudorange determination.